Process for the recovery of purified sodium



Feb. 8, 1966 K. ZIEGLER E PROCESS FOR THE RECOVERY OF PURIFIED SODIUM Filed June 27, 1962 mu m. w 7

NaAI Et IOO% INVENTORS KARL ZIEGLER HERBERT LEHMKUHL WOLFRAM GRIMME BY Ami/W;

ATTORNEYS.

plate.

United States Patent Karl Ziegler Filed June 27, 1962, Ser. No. 208,664 Claims priority, application Germany, June 30, 1961,

12 Claims. ici. 204-68) This invention relates to a process for the recovery of purified sodium.

German Patent No. 1,114,330 discloses a process in which metallic sodium is produced electrolytically with the use of electrolytes of the general formula MeAlR R'. In this formula, Me is sodium or a mixture of sodium and potassium, R is an alkyl radical, R is hydrogen, an alkyl, alkoxy or aroxy radical. Readily available and particularly recommended electrolytes for this process are sodium-aluminum tetraethyl alone or in mixture with potassium-aluminum tetraethyl, preferably further modified by addition of a small amount of a sodium or potassium-alkoxy aluminum triethyl. In this process of purifying or recovering sodium, use is made of anodes which contain sodium metal and cathodes, which are inert to the sodium metal deposited at the cathode. Due to the electrolysis process, sodium metal is dissolved out of the anode material, migrates through the electrolyte and is precipitated as purified metal at the cathode and may be withdrawn at the cathode. The temperatures used in this process are preferably elevated sufficiently that the sodium metal is present in liquid form at the cathode and may be withdrawn from the cell in this form. The substantial-1y most important application of this electrolytic recovery of sodium is the separation of sodium metal from sodium amalgam which is used as the anode material or for the electrolytic refining of raw sodium to form a particularly pure product.

The patent mentioned above gives several teachings for the practical performance of such an electrolysis. It is particularly important in practice that the sodium is precipitated in molten form and as a continuous layer at the cathode which consists of a suit-able other metal, e.g. copper. A favorable course of sodium precipitation is established in most cases by electrolyzing for some time on a bright copper plate serving as the cathode. The individual droplets initially precipitated run together to form a continuous sodium film in which the sodium flows down at the cathodes which are preferably vertical or at least inclined. A special trick taught by the patent men- 'tioned above is to provide a fabric of insulating mate-rial, e.g. a fine-meshed net of glass fibers, before the cathode It has been found very surprisingly that it is possible by means of such a net of insulating material to maintain the molten sodium in suspension within the electrolyte and to prevent an undesirable sinking-down of the cathodically deposited sodium towards the anode thereby eliminating the possibility of an undesirable formation of short circuit bridges and consequently of troubles in the electrolysis.

In practical operation, electrolytic cells of this type have to meet high requirements with respect to the useful service life. The profitableness of the process largely depends on :how often trouble, e.g. by inner short circuits, must be expected. After troubles of this kind, the cells must be disassembled completely, the electrolyte and the cathodes must be pun'fied, and it may even be necessary to fill in a fresh electrolyte. Only then the cell will again operate satisfactorily. When considering the fact that the electrolyte is very sensitive to air, it is obvious that such restoration involves rather disagreeable and costly operations.

Continuous experiments run over a period of many months have shown that the process described has still certain deficiencies in continuous operation of an electrolytic cell. Although the process of the German patent mentioned above operates satisfactorily over relatively long periods of time, troubles must be expected in case of very long operation periods. The electrolytes described tend to precipitate traces of aluminum in addition to predominantly sodium, and then growth or clusters of spongy mixtures of aluminum and sodium occur occasionally. Moreover, met-allization phenomena occur at the safety net described of insulating material when effecting the electrolysis for very extended periods of time with the use of the electrolytes mentioned so far, i.e. the net may become coated with a thin aluminum layer. However, in this case it loses the electrically insulating properties and sodium penetrates to the front side of the net so that the latter does no longer serve its purpose. Troubles of this kind may occur as early as after a few weeks of uninterrupted operation. This difiiculty is of particular importance when using mixed electrolytes of potassiumand sodium-aluminum-organic complex compounds. It is known that when electrolyzing these mixtures which normally appear advantageous from general points of view, sodium is exclusively deposited at the cathode down to a minimum content of about 20% of the sodium compound. Thus, near the cathode, the electrolyte becomes poor in sodium so that "troubles may occur unless constant very good intermixing of the electrolyte components is taken care of. Nevertheless, difficulties may be encountered, particularly with high current densities and, above all, when using the so-called net cathodes, because equalization of concentration within the electrolytes is aggravated within the filaments of the net itself. The individual filament of the net of a net cathode is a thread spun from many fibrillae having corresponding capillary interstices which become completely saturated or soaked with electrolyte. The portions of electrolyte which are bound in these interstices and subjected to the direct electrolytic decomposition mix only diificultl y with further portions of the electrolyte bath. Thus, it appears that this may involve a serious cause of undesirable troubles.

It is urgently desirable for all of these reasons to develop an electrolyte which, while having a correspondingly high conductivity in the prior art process mentioned above, contains only sodium as cations and which does not give rise to the troubles described above.

The invention consists in the finding that cells for the electrolytic recovery of sodium from an anode material which contains sodium metal, e.g. from sodium amalgam or raw sodium, can be operated for many months without any trouble when using a mixture of sodium-aluminum tetraethyl and sodium-aluminum tetramethyl as the electrolyte. The mole ratio of the components of this mixture is preferably about 1:1 but may deviate to both sides by a certain amount. The limits are at 30 to mole percent NaAl(CH ).A and 70 to 25 mole percent of NaAl C2H5) 4.

The conductivity of an exactly equimolar mixture of the two complex compounds at 130 C. is 10 10 ohms cmf i.e. the conductivity is very good. Moreover, the mixture of sodium-aluminum tetramethyl and sodium-aluminum tetraethyl melts at very low temperatures with the unusually low eutectic of 83 C. leading to completely liquid electrolyte mixtures already at temperatures in the neighborhood of C.

The accompanying drawing indicates these relationships. In the drawing:

The figure is a plot of melting points for mixtures of NaAlEL and NaAl(CH In practical operation, the. electrolytes are preferably mixed with a small amount of the corresponding sodiumalkoxy aluminum trialkyl'compound. A few percent are sufficient so that the conductivity is reduced to an unimportant extent only. This measure reliably protects the cathodically deposited sodium from co-deposition of aluminum. The alkoxy triethyl ortrirnethyl compoundmay be added most conveniently. by adding a corresponding amount of any alcoholto the. mixture .of the two tetraalkyl complexes. The temperatures to be maintained during el ctrolysis must range above the melting point of sodium. Most preferred are temperatures: between about 100 and 120 C., but temperatures up to about 160 C. may be used.

The preparation of sodium-aluminum tetraethyl which is one component of the process of the invention is sufficiently described in literature. The other component,

i.e. sodium-aluminum tetramethyl, may, for example, be

prepared by one of the processes described below.

EXAMPLE 1 50 grams (=2.18 gram atoms) of sodium are heated to 180 C. in a dry 1 liter three-necked fiask filled with inert gas .and equipped with a stirrer, a reflux condenser and a dropping funnel. While stirring, 100 mls. (=96 gms. =l.04 moles) of Al(CH Cl-are dropped onto the molten sodium. When all of the aluminum dimethyl chloride. has been added, the pulverulentreaction mixture is allowed to cool while stirring. Then 300 ml. of dry diethyl ether are added and the mixture is stirred for about 20 minutes. After having stopped the stirrer, the solid products (NaCl and aluminum) are allowed to settle, the clear ether solutiqn of NaA1(CH is with-i drawn by siphoning, and theresidue is extracted three;

times with 200ml. of ether. After removal of the ether by distillation and drying under a vacuum of mm. Hg the combined solutions give 55 gms. (=0.5 moles) of NaAl(Cl-I following values:

Found: Na, 20.95; Al, 24.6; CH 54.2. Calculatedz.

Na, 20.9; A1, 24.6; CH 54.6. The yield is 96.5%.

EXAMPLE 2 NaAl(CI-I can be prepared in the same. manner as described in Example 1 by allowing 48.3 'gms.v (=0.67

mole) of Al(CH3) to drop into 11.5 gms. (=0.-5 gram atoms) of molten sodium at 180-200 C. The yield andpurityof the product are similarly good as those in Example i1.

' EXAMPLE 3 9.7 gins. (=58.4 millimoles) NaAlEt; are dissolved. in 50. ml. benzene at 70 C. while stirring. A solution of 5.6 gms. (=78 millimoles) Al(CH in ml. benzene is dropped into the solution. A white insoluble precipi-i tate is formed immediately. The reaction mixture .is

stirred for another hour and then the precipitate is filtered tetramethyl to vmolten sodium-aluminum tetraethyl with.

exclusionof air or to dissolve both substances inair-free and dry diethyl ether or tetrahydrofuran and to evaporate these solutions to dryness. It is also possible to combine the production of the mixtures directly with the preparation otthe alkali-aluminum tetraalkyl compounds. This is described in the following examples.-

Analysis of the substance gives the k EXAMPLE 4 The procedure is initially the same as that describedin Example 3. However, the precipitate of sodium-alumi-v num tetrarnethyl is not filtered ofl? but 1.9 gms. of sodium hydride which ispreferablyin the form of a 20% suspension in a mineral oil is addedto the reaction'mixture which still containsfree aluminum triethyl After stirring.

duced zby'the sodium hydride, the lower layerxbeingia.

mixture according to the invention comprising sodiumaluminum te'traethyl and sodium-aluminum;tetrarnethyl in a mole ratio of.4: 3..

' EXAMPLE 5 In the manner described'in Example 1, a mixture of 48 gms'. (=05 mole) Al(CH C-l and 60 gms. (0.5

mole).Al(C H Cl is allowedto drop at about 150 C. onto 50 gms. (=2.18 gram atoms) of molten sodium while stirring. After cooling, 300 ml. of dry diethyl ether are added to the reaction mixture which is then stirred about 20. minutes.

settle. The clear ether; solution is siphoned otf andthe residue is againextracted with about200 ml. of ether: The combined ether solutionmgive 69 gms. of a mixture of NaAl(CH andNaAKC HSM in a mole ratio of 1:1 afterl1avingdistilled.olfthe' ether and dried the residue under, a vacuum of 10- mm. Hg; This reaction product melts at 83 C. The yield is substantially quantitative.

EXAMPLE '6 The equirnolar. mixture, of NaAKCHgM and NaAl. (C H can be prepared in a manner; similar tothat dew scribed in Example 5 by allowing an equimolar mixture of Al(CH and AI(C H (0.67 mole each) to drop onto 23 gms. (=1 gram atom) of molten sodium. .The reaction mixture isprocessed in a manner analogous'to that described in Example 5.;

EXAMPLE 7 457 gms."(=5 miles) Al(CH Cl are heated to about:

to C. in. a dry 1000iml; flask providedwitha stirrer and reflux condenser and 'filled with inert gas;

Then 600 gms. of a 20% NaH-sus'pension inparafli-n oil (containing 'gm's. NaH (:5 moles)) are carefully added by siphoning, following which the reaction mix-2 ture :isstirredfor aboutl houruntilhalogen is no longer contained in the solution, After cooling, the reaction mixture isqsepa'rated from precipitated:NaCl by centrifuging and filled'into'a2 liter autoclave while taking the known precautionary measures usual for handling airand water-sensitive organometallic' compounds. The contents of the autoclave are heated to about;60'70 C.

while shaking or; rotating the reaction vesseletfollowing which 10 to 20. atmospheres of ethylene'are introduced under pressure. The initially rapid pressure drop necessitates frequent supply of addit'ionaliethylene .until the pres- Afterhaw sure remains constant :after about 451 minutes. ing vented the excess; ethylene,1.600: gms. of the20% NaH suspension (containing: 120 :igms: NaH=0i5 mole) are' introduced into the autoclave andthe autoclave isshaken or tumbled for about :15 to 30 minutesat'SO to l00 Cguntil' all of'the-NaI-I has dissolved, whichisverified-by I sampling after 30 minutes. Then 20-atmospheres of ethylene are introduced under pressure and the contents: of the autoclave are heated to .160 C. T'ne-ethylenecon- After having stopped the stirrer, the solid products, i.e., NaCl and aluminum, are allowed to.

surned by the reaction is made up by repeatedly introducing ethylene under pressure. The pressure drop ceases after 2 to3 hours and the reaction is completed.

After having vented excess ethylene, the contents of the autoclave, while still liquid, i.e. at a temperature above 90 C., are siphoned into a dry 2 liter flask filled with an inert gas. At 90 C., the reaction product is a liquid system comprising two phases. The upper layer is paraflin oil which is substantially free from organometallic compounds and siphoned oil from the lower phase. The lower layer is a mixture of NaAl(CH and NaAl (C H in a mole ratio of 1:1 which solidifies at 80 C. with formation of crystals. The yield of this mixture is 650 gms. (95% of the theoretical).

EXAMPLE 8 Into a 10 liter vessel with stirrer, there are placed a suspension of 1 kg. aluminum grit and a mixture of AlEt Cl and AlMe Cl in a molar ratio of 1:1 in amount sufficient that the mixture is still well stirrable. The reaction vessel is heated to 1051l5 C. and an equimolar mixture of ethyl chloride and methyl chloride in liquid form (from an inverted stock bottle) is introduced through a capillary extending down to the bottom of the vessel. It is also possible to introduce ethyl chloride and methyl chloride in equal amounts separately through two capillaries. The addition of the alkyl halides is effected at a rate suflicient that the temperature is maintained between 105 C. and 115 C.

Alkyl chloride in amount of between 350 and 7 grams is taken up within one hour. 3 kgs. of the mixture of alkyl halides and all of the aluminum charged are reacted within about 6 hours. The conversion of the resultant mixture of methyl and ethyl aluminum sesquichlorides into Al(CH Cl and Al(C H Cl or Al(CH (C H )Ol is effected as follows: 425 gms. of sodium are molten in a second liter reaction vessel. The reaction mixture of the alkyl aluminum sesquichloridex is allowed to drop onto the sodium at about 110 C. While stirring. After all of the mixture has been added, stirring is continued for about 30 minutes. Following this, the resultant is distilled oil from the sodium chloride and aluminum at a bath temperature which must be increased to 200 C. towards the end of the distillation.

There are obtained 2.5 kgs. Al (CH (C H )Cl, i.e. 83% of the theoretical yield.

The complex mixture NaA-l(CI-l NaAl(C H is prepared in a manner analogous to that of Example 1 by allowing the reaction mixture having the composition Al(CH )(C H )Cl and obtained in Example 8 to drop onto the corresponding amount of molten sodium. There is obtained the 1:1 molar mixture NaAl (CI-1 NaAl(C H in a good yield.

EXAMPLE 9 The electrolytic cell used consists of a cylindrical and internally enamelled steel kettle which contains at the bottom raw sodium to be refined as a melt. Suspended in the kettle is a cylinder of enamelled sheet steel of somewhat smaller diameter, which is open at both ends and has horizontally tightened across the lower opening a wide-meshed glass fiber fabric having a mesh size of 1 to 3 mm. The net is arranged at a distance of 3 to 5 mm. above the surface of the liquid sodium. Arranged closely above the net is a net of copper or iron wire as the cathode.

The electrolysis temperature is 150 C. A mixture of sodium-aluminum tetraethyl and tetramethyl in a 1:2 molar ratio is used as the electrolyte. The level of the molten electrolyte must be above the upper edge of the suspended cylinder, so that the sodium deposited at the cathode is surrounded by the electrolyte from above and below. An electrode current density of 20 a./dm. can be maintained at a terminal voltage of 1.1 v. The cathodically formed sodium collects above the glass fiber net and may be drained from this space from time to time. Care is taken by the addition of raw sodium during the electrolysis that the distance between the anode and cathode is kept constant.

Under the conditions described above, the electrolytic refining of raw sodium can be carried out continuously for months without any trouble. Test runs were operated for more than six months. In contrast, test runs effected with electrolytes containing only ethyl groups had to be discontinued after 3 to 4 weeks due to troubles in the sodium deposition. The same difierence is suitability of the electrolyte systems was also found in test runs which were elfected for an extended period of time with the use of sodium amalgam as the anode (see Example 10 below).

The yield of sodium is 23 grams per 26.8 ampereshours, and 23 grams of Na were dissolved anodic-ally by the same amount of current. The yield is EXAMPLE 10 The procedure is the same as that described in Example 9, except that the same volume of 1% sodium is substituted for the molten raw sodium. Electrolysis is effected at C. with a current density of 30 -a./dm. at a terminal voltage of 1.7 v. The sodium deposited at the cathode collects as a continuous liquid layer above the net of glass fiber fabric. To remove the joulean heat and for mixing the electrolyte, a greater electrolyte stock is recirculated through the electrolytic cell while maintaining the liquid in the electrolytic cell at a constant level. This is easily accomplished by allowing an equal volume of electrolyte per unit time to flow from a stock vessel into the space between the anode and cathode as that pumped back into the stock vessel from a point opposite of the inlet opening. The amount of sodium deposited cathodically after 185 amperes-hours is grains. The Na content of the initial Na amalgam which was 1% has been reduced to about 0.2%. The amalgam is preferably removed from the cell and may be used for another electrolysis after having been concentrated to 1% Na.

In this experiment, the same current density (30 a./ (1m?) with about the same voltage (1.5-1.7 v.) is achieved as it would be obtained with an electrolyte mixture of 80% KAlEt and 20% NaAlEt under otherwise identical conditions.

EXAMPLE 11 The procedure is the same as that described in Example 10 except that an electrolyte mixture of NaAl(C H and NaAl(CH in a 1:1 molar ratio is used. A terminal voltage of about 1.5 to 1.7 v. is required at a current density of 20 a./dm.

What is claimed is:

1. A process for the recovery of purified sodium which comprises passing an electrolysis current between a cathode and an anode through an electrolyte containing a mixture of sodium aluminum tetramethyl and sodium aluminum tetraethyl, said anode being a sodium metal containing electrode and said cathode being inert to the cathodically deposited sodium, to electrolyze sodium from the anode and deposit sodium at the cathode, the sodium electrolyzed at the anode replacing in the electrolyte sodium deposited at the cathode, and recovering the purified sodium so formed.

2. The process of claim 1 wherein said electrolyte comprises from 30 to 75 mole percent of NaAl(CH and from 70 to 25 mole percent of NaAl(C H 3. The process of claim 1 wherein said electrolyte comprises about equivalent amount of NaAl(CH and NaAl(C H 4. The process of claim 1 wherein said electrolyte additionally contains small amounts of a corresponding alkoxy complex compound.

5. The process of claim 1 which includes effecting said cathode space and saidanode-space separated by a diaphragm formed of an insulating material.

6; The process of claim 1 wherein the sodium aluminum tetramet-hyl component of said electrolyte is prepared by reactingaluminumtrimethyl with sodium metal.

7. The process of claim 1 wherein" the sodium aluminum tetramethyl component of said electrolyte is pr-e1 pared'by reacting a member selected from the-group, consisting of AI(CH X, Al(CH )X wherein X is halogen, and'mixtures thereof with sodium'metal.

8. The process of claim 1 wherein thesodi-um aluminum' tetramethyl coniponent of said electrolyte is pre-' pared by reactingaluminurn trimethyl with a sodium aluminum tetraalkyl compoundwhereinalkyl contains at least two carbon atoms;

9; The process of claim -1 wherein the electrolyte is maintained at a temperature of betweenwabout 100 and a 11. The process according; to clairn -l wherein the electrolysis is effectedwith said anode as a lowermost la er;

the electrolyte as the-middle layer 'and'the "cathodically'f deposited sodium kept insuspension by, a widemeshfnet' of "insulating material as theflupp r most layer'.-

12. Process according to'claim 11,; wherein the; net comprises threads formed of filaments.

References Citedabytlie Examiner-f UNITED STATES PATENTS JOHN H. MACK,- Primary Examiner;

JOHN R. SPECK,,1Examiner. 

1. A PROCESS FOR THE RECOVERY OF PURIFIED SODIUM WHICH COMPRISES PASSING N ELECTROLYSIS CURRENT BETWEEN A CATHODE AND AN ANODE THROUGH AN ELECTROLYTE CONTAINING A MIXTURE OF SODIUM ALUMINUM TETRAMETHYL AND SODIUM ALUMINUM TETRAETHYL, SAID ANODE BEING A SODIUM METAL CONTAINING ELECTRODE AND SAID CATHODE BEING INERT TO THE CATHODICALLY DEPOSITEDD SODIUM, TO ELECTROLYZE SODIUM FROM THE ANODE AND DEPOSIT SODIUM AT THE CATHODE, THE SODIUM ELECTROLYZED AT THE ANODE REPLACING IN THE ELECTROLYTE SODIUM DEPOSITED AT THE CATHODE, AND RECOVERING THE PURIFIED SODIUM SO FORMED. 